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Formula: Cu4(SO4)(OH)6 sulphate with hydroxyl
Specific gravity: 3.97
Hardness: 3½ to 4
Streak: Green to light green
Colour: Emerald green
Solubility: Moderately soluble in hydrochloric, sulphuric and nitric acid
Environments:
Brochantite occurs as a secondary mineral in the oxidation zone of
high temperature copper ores in acid conditions. When the carbonate concentration
is similar to the atmospheric value, at successively higher values of pH (less acid conditions),
chalcanthite is replaced by antlerite
then brochantite then malachite. Although the environment for brochantite
is at a higher pH than chalcanthite and
antlerite, it is still on the acid side of neutral. In the UK brochantite is
relatively common in low carbonate environments such as the Caldbeck Fells, but much less so in the more carbonate-rich
Pennines
(JRS 18.12, 13).
Localities
At the Mount Kelly deposit, Gunpowder District, Queensland, Australia, the copper
ores overlie primary zone mineralisation consisting of
quartz-dolomite-sulphide veins hosted
in siltstone and schist.
Brochantite is widespread throughout the deposit; it formed late in the paragenesis and was found coating
azurite, chalcopyrite,
covellite, cuprite,
goethite, hematite,
malachite and pyrite
(AJM 22.1.24).
At Jazkazgan, Karaganda Oblast, Kazakhstan, brochantite pseudomorphs after
bornite
have been found
(KL p191).
Alteration
Primary chalcopyrite
readily alters to the
secondary minerals bornite,
covellite and brochantite.
antlerite and water to brochantite and sulphuric acid
4Cu3SO4(OH)4(s) + 2H2O(l) → 3Cu4SO4(OH)6(s)
+ H2SO4(aq)
(JRS 18.12, 13)
The Activity-pH diagram below was calculated at 298.2 K for some arsenates
and sulphates for constant activity (roughly equivalent to concentration) of H2AsO4- in solution,
over a range of values of pH and of SO42- activity
(MM 52.689).
The mineral formulae are:
chalcanthite: Cu(SO4).5H2O
antlerite: Cu2+3(SO4)(OH)4
brochantite: Cu4(SO4)(OH)6
olivenite: Cu2(AsO4)(OH)
cornubite: Cu5(AsO4)2(OH)4
clinoclase: Cu3(AsO4)(OH)3
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